A person's circulatory system includes both systemic and pulmonary circulation. Pulmonary circulation supplies the lungs with blood flow, while the systemic circulation takes care of all the other parts of the body. The heart serves as a pump that circulates the blood, while blood vessels act as the conduits that deliver blood to tissue. Both the pulmonary and systemic circulatory systems are made up of arteries, arterioles, capillaries, venules and veins. The arteries take the blood from the heart, while the veins return the blood to the heart
Blood pressure is defined as the force exerted by the blood against any unit area of the vessel wall. The measurement unit of blood pressure is millimeters of mercury (mmHg). Pulmonary and systemic arterial blood pressures are pulsatile, having systolic and diastolic blood pressure values. The highest recorded pressure reading in a cardiac cycle is called systolic blood pressure, which results from the active contraction of the ventricle. Although the arterial blood pressure and indeed flow in the arteries is pulsatile, the total volume of blood in the circulation changes little over a cardiac cycle. The lowest arterial pressure reading in a cardiac cycle is called diastolic blood pressure which is maintained by the resistance created by the smaller blood vessels still on the arterial side of the circulatory system (arterioles). Stated another way, the systolic blood pressure is defined as the peak pressure in the arteries, which occurs near the beginning of a cardiac cycle, where a cardiac cycle can be said to begin when blood is ejected from the ventricles. In contrast, the diastolic blood pressure is the lowest pressure, which occurs at the resting phase of the cardiac cycle. The pulse pressure reflects the difference between the maximum and minimum pressures measured (i.e., the difference between the systolic blood pressure and diastolic blood pressure). The mean arterial blood pressure is the average pressure throughout the cardiac cycle.
Arterial pulse pressure, such as mean arterial blood pressure (MAP), is a fundamental clinical parameter used in the assessment of hemodynamic status of a patient. Mean arterial blood pressure can be estimated from real pressure data in a variety of ways. Among the techniques that have been proposed, one is presented below. In this formula, SBP is the systolic blood pressure, and DBP is diastolic blood pressure.
  MAP  =                    (                  SBP          +                      2            ⁢                                                  ⁢            DBP                          )            /      3        =                            1          3                ⁢                  (          SBP          )                    +                        2          3                ⁢                  (          DBP          )                    
Systolic blood pressure and diastolic blood pressure can be obtained in a number of ways. A common approach is to use a stethoscope, an occlusive cuff, and a pressure manometer. However, such an approach is slow, requires the intervention of a skilled clinician and does not provide timely readings as it is a measurement at only a single point in time. While systolic blood pressure and diastolic blood pressure can also be obtained in more automated fashions, it is not always practical to obtain measures of pressure using a cuff and pressure transducer combination, especially if the intention or desire is to monitor systemic arterial blood pressure on a chronic basis.
Another approach for obtaining measures of arterial blood pressure is to use an intravascular pressure transducer. However, an intravascular device may cause problems, such as, embolization, nerve damage, infection, bleeding and/or vessel wall damage. Additionally, the implantation of an intravascular lead requires a highly skilled physician such as a surgeon, electrophysiologist, or interventional cardiologist.
Plethysmography, the measurement of volume of an organ or body part, has a history that extends over 100 years. Photoplethysmography (PPG) uses optical techniques to perform volume measurements, and was first described in the 1930s. While best known for their role in pulse oximetry, PPG sensors have also been used to indirectly measure blood pressure. For example, non-invasive PPG sensors have been used in combination with in an inflatable cuff in a device known as Finapres. U.S. Pat. Nos. 4,406,289 (Wesseling et al.) and 4,475,940 (Hyndman) are exemplary patents that relate to the Finapres technique. The cuff is applied to a patient's finger, and the PPG sensor measures the absorption at a wavelength specific for hemoglobin. After the cuff is used to measure the individual's mean arterial blood pressure, the cuff pressure around the finger is then varied to maintain the transmural pressure at a constant predetermined pressure as determined by the PPG sensor. The Finapres device tracks the intra-arterial blood pressure wave by adjusting the cuff pressure to maintain the optical absorption constant at all times.
There are a number of disadvantages to the Finapres technique. For example, when there exists peripheral vasoconstriction, poor vascular circulation, or other factors, the blood pressure measured in a finger is not necessarily representative of central blood pressure. Further, maintaining continuous cuff pressure causes restriction of the circulation in the finger being used, which is uncomfortable when maintained for extended periods of time. Accordingly, the Finapres technique is not practical for chronic use. Additionally, because of the need for a pneumatic cuff, a Finapres device can not be used as an implanted sensor.
Simple external blood pressure monitors also exist, but they do not offer continuous measurement and data logging capability. These devices can be purchased at a drug store, but patient compliance is required to make regular measurements and accurately record the data. Additionally, portable external miniature monitors that automatically log blood pressure data exist, but these devices can only store a day or so of data and require clinician interaction to download and process the measured data.
As is evident from the above description, there is the need for improved systems and methods for monitoring arterial blood pressure, including systolic blood pressure, diastolic blood pressure and mean arterial blood pressure.
Electromechanical delay (EMD) is the time delay between onset of ventricular electrical activation and mechanical ejection of blood from the heart. This delay is partly due to the time required for the contractile elements of muscles to stretch the series elastic components. EMD is believed to be affected by conduction abnormalities, myocardial contractility and cardiac diseases, including but not limited to heart failure (HF), mitral stenosis, and hypertension. Accordingly, monitoring EMD can be useful for monitoring conduction abnormalities, myocardial contractility and cardiac diseases.